1. Field of the Invention
The present invention relates to a cathode active material for a lithium rechargeable battery, a manufacturing method thereof, and especially, a lithium rechargeable battery having a superior cyclic characteristic.
2. Description of Related Art
Along with the recent rapid progress in the field of domestic appliances toward portable and cordless, lithium ion rechargeable batteries have come into practical use as power sources for compact electronic devices such as laptop computers, portable telephones and video cameras. With regard to the lithium ion rechargeable batteries, Mizushima et al. reported in 1980 the usefulness of a lithium cobaltate as a cathode active material for a lithium ion rechargeable battery (Related Art 1), and since then, active research and development efforts have been made on lithium-based composite oxides, resulting in numerous proposals on the subject to date.
However, a lithium rechargeable battery that uses a lithium cobaltate has a cyclic characteristic degradation problem due to elution of cobalt atoms.
There are also proposals for using a lithium-cobalt based composite oxide as a cathode active material, the lithium-cobalt based composite oxide being obtained by partially substituting Co atoms of a lithium cobaltate with Mg atoms (for example, Related Arts 2-4).
In Related Arts 2-4, a magnesium oxide or a magnesium carbonate is used as an Mg source of raw materials for lithium-cobalt based composite oxides. Satisfactory cyclic characteristic is hard to achieve in a lithium rechargeable battery that uses a lithium-cobalt based composite oxide containing so-obtained magnesium atoms (hereafter, also noted as Mg atoms) as a cathode active material.
The inventors of the present invention previously proposed to use a lithium-cobalt based composite oxide as a cathode active material for a lithium rechargeable battery, the lithium-cobalt based composite oxide using a magnesium salt (such as a halide of MgF2, a magnesium phosphate and a magnesium hydrogen phosphate) as an Mg source (for example, Related Arts 5-7).    [Related Art 1] Material Research Bulletin, vol. 15, pages 783-789, 1980    [Related Art 2] Japanese Patent Laid Open Publication No. HEI 5-54889, pages 1 and 8    [Related Art 3] Japanese Patent Laid Open Publication No. 2000-11993, pages 2 and 3    [Related Art 4] Japanese Patent Laid Open Publication No. 2004-79386, pages 2 and 7    [Related Art 5] Japanese Patent Laid Open Publication No. 2003-221235    [Related Art 6] Japanese Patent Laid Open Publication No. 2004-339032    [Related Art 7] Japanese Patent Laid Open Publication No. 2006-169048
The inventors of the present invention noticed in above-mentioned Related Arts 5-7 or during further studies that, when a metal compound, selected from a group consisting of a halide, a phosphate, a hydrogen phosphate and a sulfate of either Mg or Al, is used as a source of either Mg or Al, metal atoms preferentially exist as oxides of the metal on surfaces of particles of a sintered lithium transition metal composite oxide, due to an action of an anion component of the metal compound, and inhibit elution of cobalt ions that affect the cyclic characteristic. Therefore, a lithium rechargeable battery that uses the lithium transition metal composite oxide as a cathode active material shows a superior cyclic characteristic. Further improvement in performance of a lithium rechargeable battery is expected.
On the other side, when the lithium transition metal composite oxide is industrially produced, an anionic material reacts with a sintering container (such as a pot, an oven and a pod), which is made of alumina or silica and is used in a sintering process, and attaches to the interior of the sintering container, thereby reducing the productivity and making it difficult to obtain a product having stable quality.